1. Field of the Invention
The present invention relates to a carrier core material for an electrophotographic developer, a carrier, and an electrophotographic developer using the carrier, in a two-component electrophotographic developer used in copiers, printers and the like.
2. Description of the Related Art
Two-component electrophotographic developers used in electrophotographic methods are formed from a toner and a carrier. The carrier acts as a carrier substance that is mixed with the toner by stirring in a developing box to impart a desired charge to the toner and transport the charged toner to the surface of a photoreceptor to form an electrostatic latent image. Carrier remaining on the developing roll which is supported by magnets after forming the toner image returns back into the developing box, and is then mixed and stirred with new toner particles for reuse over a certain time period.
Unlike one-component electrophotographic developers, for these two-component electrophotographic developers, the carrier is stirred with the toner particles to impart desired charge properties to the toner particles and has a function of transporting the toner, and controllability in developer design is good. Therefore, two-component electrophotographic developers are especially widely used in full color developing machines for which high image quality is demanded and in high-speed machines for which the reliability and durability of image sustainability are demanded.
In such a two-component electrophotographic developer, to obtain high image quality, ferrite particles, such as Cu—Zn ferrite and Ni—Zn ferrite, may be used as a carrier instead of oxide-coated iron powder and resin-coated iron powder. Ferrite carriers using such ferrite particles have many advantageous properties for obtaining high image quality, such as usually being more spherical than conventional iron powder carriers and having adjustable magnetic properties. Further, a resin-coated ferrite carrier which has such ferrite particles as a core material and is coated with various resins, has improved abrasion resistivity, durability and the like, and has an adjustable volume specific resistivity.
However, recently, environmental regulations have become more strict, and the use of metals such as Ni, Cu, and Zn is now avoided. Thus, there is a need to use metals which comply with the environmental regulations.
Examples of uses of metals which comply with the environmental regulations include the conventionally-used iron powder carriers and magnetite carriers. However, with these carriers, it is difficult to obtain the image quality and life of the above-described ferrite carriers.
As a ferrite which complies with the environmental regulations, Li—Mn ferrite has been proposed. However, it is pointed out that Li is easily affected by the surrounding environment, such as temperature and humidity, so that its properties greatly change.
Japanese Patent Laid-Open No. 7-333910 discloses a ferrite carrier for an electrophotographic developer wherein a part of the Li ferrite is substituted with at least one selected from the group consisting of alkaline earth metal oxides. Specific examples thereof are mentioned in the examples as Li—Mg ferrite carriers and Li—Mg—Ca ferrite carriers. This Japanese Patent Laid-Open No. 7-333910 describes that a carrier for an electrophotographic developer can be obtained which can maintain durability equal to or better than conventional ferrite particles, and which has excellent stability against the surrounding environment. However, this carrier for an electrophotographic developer suffers from the problem of having low scattered matter magnetization due to sintering unevenness.
In Comparative Examples 7, 12, and 17 of Japanese Patent Laid-Open No. 7-333910, a Li—Mn ferrite carrier is described. These Comparative Examples 7, 12, and 17 are described as having a large changes in charge amount under environmental fluctuation, and a large amount of scattered matter.
Japanese Patent Laid-Open No. 9-6052 describes a ferrite carrier for an electrophotograph which includes a fixed amount of V2O5 and Bi2O3 in Li ferrite and Li—Mn ferrite, whereby abnormal crystal particle growth is suppressed, and as a result, there is little toner contamination and a long life is obtained.
Sample No. 8 of Japanese Patent Laid-Open No. 9-6052 describes a Li—Mn ferrite carrier, in which contamination from the toner resulting from changes in the charge amount occurs.
Japanese Patent Laid-Open No. 9-236945 describes a two-component developer formed from a Li—Mn ferrite carrier, which is coated with resin on its surface and which has a specific volume specific resistivity, and a specific toner, wherein a high quality image can be obtained which is stable for a long period of time.
However, such Li—Mn ferrite carriers suffer from the problem that usually resistivity is high, and that resistivity is reliant on electric field strength. In carriers for an electrophotographic developer, just having a resistivity in a specific electric field strength in a specific range is insufficient. While developing bias changes in development which employs electrophotography, a resistivity is required which constantly ensures that no carrier beads carry over occurs even if the developing bias changes. Moreover, it is preferred to obtain an image density which is stable even if the developing bias changes. For this reason, it is necessary to ensure that there are no large changes in resistivity between low bias and high bias.